This invention relates to missile tracking systems and in particular, but not exclusively to tracking systems which employ a solid state frame transfer charge coupled device image sensor (FTCCD).
A known form of such a system comprises a camera for forming an electrical video signal representative of a viewed scene containing a target and the flare of a missile being guided towards the target, the video signal being passed to electronic guidance apparatus which guides the missile within the field-of-view of the system, and also being passed to a display monitor so that an operator can maintain the system aimed at the target. The target itself and the viewed scene in general may be quite dull while the missile flare will usually be very bright. Also, the scene may contain some discrete fairly bright features, notably cloud edges and the like. In order to give the operator the best possible view of the target on the display monitor and to provide the best possible signal-to-noise ratio of the system in the face of the constant, ie sensitivity independent, base level of noise generated within the image sensor of the camera, the camera sensitivity is best adjusted to be as high as possible by reference to the general brightness of the viewed scene. However, because of the limited dynamic range of available image sensors, this will almost certainly mean that the missile flare image is well above the saturation limit of the camera so that, as far as the guidance apparatus is concerned, the apparent brightness of the aforementioned fairly bright scene features may approach or even equal that of the missile flare and hence may be confused with it.
The applicants have proposed in UK Patent Application No. 8431568, to which reference is directed, a system in which the sensitivity of an image sensor is controlled differently respectively during a first and a second plurality of fields alternately one with another.
The sensitivity during the respective periods may be respectively high and low so that, in effect, a greater dynamic intensity range of the sensor is obtained. In a missile guidance application the higher sensitivity portions are fed to a display to give an operator a good view of the target, whilst lower sensitivity portions are passed to a missile guidance unit so that this "sees" substantially only the missile flare and hence is not confused by cloud edges and the like.
The above system whilst possessing many advantages over the prior art is disadvantaged by the fact that the brightness of the missile flare may obscure the target when the missile is actually on the line of sight between the target and the tracker and moreover, during target tracking fields the sensitivity will be high and thus the image may spread.